Medical implants for greatly varying uses are known in the art. A shared goal in the implementation of modern medical implants is high biocompatibility, i.e., a high degree of tissue compatibility of the medical product inserted into the body. Frequently, only a temporary presence of the implant in the body is necessary to fulfil the medical purpose. Implants made of materials which do not degrade in the body are to be removed again, because rejection reactions of the body may occur in long term even with highly biocompatible permanent materials.
One approach for avoiding additional surgical intervention is to form the implant entirely or in major parts from a biodegradable (or biocorrodible) material. The term biodegradation as used herewith is understood as the sum of microbial procedures or processes solely caused by the presence of bodily media, which result in a gradual degradation of the structure comprising the material. At a specific time, the implant, or at least the part of the implant which comprises the biodegradable material, loses its mechanical integrity. The degradation products are mainly resorbed by the body, although small residues being in general tolerable.
Biodegradable materials have been developed, inter alia, on the basis of polymers of synthetic nature or natural origin. Because of the material properties, but particularly also because of the degradation products of the synthetic polymers, the use of biodegradable polymers is still significantly limited. Thus, for example, orthopedic implants must frequently withstand high mechanical strains and vascular implants, e.g., stents, must meet very special requirements for modulus of elasticity, brittleness, and moldability depending on their design.
One promising attempted achievement provides the use of biodegradable metal alloys. For example, it is suggested in German Patent Application No. 197 31 021 A1 to form medical implants from a metallic material whose main component is to be selected from the group of alkali metals, alkaline earth metals, iron, zinc, and aluminium. Alloys based on magnesium, iron, and zinc are described as especially suitable. Secondary components of the alloys may be manganese, cobalt, nickel, chromium, copper, cadmium, lead, tin, thorium, zirconium, silver, gold, palladium, platinum, silicon, calcium, lithium, aluminium, zinc, and iron.
The use of a biodegradable magnesium alloy having a proportion of magnesium greater than 90% by weight, yttrium 3.7-5.5% by weight, rare earth metals 1.5-4.4% by weight, and the remainder less than 1% by weight is known from European Patent 1 419 793 B1. The material disclosed therein is in particular suitable for producing stents.
Another intravascular implant is described in European Patent Application 1 842 507 A1, wherein the implant is made of a magnesium alloy including gadolinium and the magnesium alloy is being free of yttrium.
Stents made of a biodegradable magnesium alloy are already in clinical trails. In particular, the yttrium (W) and rare earth elements (E) containing magnesium alloy ELEKTRON WE43 (U.S. Pat. No. 4,401,621) of Magnesium Elektron, UK, has been investigated, wherein a content of yttrium is about 4% by weight and a content of rare earth metals (RE) is about 3% by weight. The following abbreviations are often used: RE=rare earth elements, LRE=light rare earth elements (La—Pm) and HRE=heavy rare earth elements (Sm—Lu). However, it was found that the alloys respond to thermo-mechanical treatments. Although these types of WE alloys originally were designed for high temperature applications where high creep strength was required, it was now found that dramatic changes in the microstructure occurred during processing with repetitive deformation and heat treatment cycles. These changes in the microstructure are responsible for high scrap rates during production and inhomogeneous properties of seamless tubes and therefore in the final product. As a consequence, mechanical properties are affected harmful. Especially, the tensile properties of drawn tubes in the process of manufacturing stents are deteriorated and fractures appear during processing. In addition, a large scatter of the mechanical properties especially the elongation at fracture (early fractures of the tubes below yield strength during tensile testing) was found in the final tube. Finally, the in vivo degradation of the stent is too fast and too inhomogeneous and therefore the biocompatibility may be worsted by inflammation process caused by a tissue overload of the degradation products.
The use of mixtures of light rare earth elements (LRE; La, Ce, Pr, Nd) and heavy rare earth metals (HRE; elements of the periodic table: Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in commercially available magnesium alloys such as WE43 rather than pure alloying elements reduced the costs and it has been demonstrated that the formation of additional precipitates of these elements beside the main precipitates based on Y and Nd further enhance the high temperature strength of the material [King et al, 59th Annual World Magnesium Conference, 2005, p. 15ff]. It could therefore be postulated the HRE containing precipitates are more stable against growth at higher temperatures because of the significantly slower diffusion rate of these elements compared to Y and Nd. Therefore they contribute substantially to the high temperature strength of WE alloys (particle hardening effect).
However, it now has been found that these HRE precipitates are causing problems when the material is used in biomedical applications, such as vascular implants (e.g. stents) or in orthopaedic implants. The HRE intermetallic particles adversely affect the thermo-mechanical processability of alloys. For example, manufacturing vascular prostheses like stents made of metallic materials usually starts from drawn seamless tubes made of the material. The production of such seamless tubes is usually an alternating process of cold deformation by drawing and subsequent thermal treatments to restore the deformability and ductility, respectively. During the mechanical deformation steps intermetallic particles cause problems because they usually have significant higher hardness than the surrounding matrix. This leads to crack formation in the vicinity of the particles and therefore to defects in the (semi-finished) parts which reduces their usability in terms of further processing by drawing and also as final parts for production of stents.
Intermetallic precipitates also adversely affect the recrystallization behaviour during heat treatments for restoring the plasticity. Impurities are known to affect grain boundary mobility strongly, depending on segregation and mobility. Further, not only the volume failures (precipitates) in the microstructure but also point failures (foreign atoms=all alloying elements) and especially RE atoms contribute to this adverse effect.
Surprisingly it now has been found that precipitation still happens to occur although the temperature regime is high enough that one would expect dissolution of all existing particles. This indicates that intermetallic phases predominantly formed with LRE cannot be dissolved during usual recrystallization heat treatment (300 to 525° C.) of the specific alloys. As a consequence, the ductility for further deformation processes or service cannot be restored sufficiently.